This is a national stage of PCT application No. PCT/FI00/00066, filed on Feb. 1, 2000. Priority is claimed on that application.
The invention relates to a method for matching the characteristic impedances of a transmission line when the transmission line is taken into a wall made of dielectric material. The invention also relates to a transmission line characteristic impedance coupler to change the characteristic impedance of a transmission line.
In certain RF structures, a signal transmission line has to be modified in terms of either dimensions or structure. One such case is a signal line feedthrough from free space into a hermetically sealed Monolithic Microwave Integrated Circuit (NIMIC) integrated circuit package. When such a feedthrough is realized in the wall of the package, the characteristic impedance changes at the interfaces of the feedthrough. That change i: caused by the change in the conductor structure, the change in the relative permittivity (xcex5r) of the material around the conductor at tie interface, and by possible ground potential planes in the vicinity of the conductor. These factors together affect the shape of the electromagnetic field on tie different sides of the interface. The change in the field shape causes part of the signal arriving at the interface to be reflected back in its direction of incidence. The ratio of the reflected signal to the signal incident upon the interface, designated as either xcfx81 or, commonly in RF technology, as S11, return attenuation, is obtained from equation (1). The smaller ratio, the better the matching of the characteristic impedance at the interface of the feedthrough.                                           S            11                    =                                                    Z                2                            -                              Z                1                                                                    Z                2                            +                              Z                1                                                    ,                            (        1        )            
where
S11=reflection coefficient,
Z1=characteristic impedance of the conductor coming to the interface,
Z2=characteristic impedance of the conductor leaving the interface.
This power loss at the interface caused by a mismatch of characteristic impedances is called reflection attenuation, equation (2).                               Γ          =                      10            ⁢                          xe2x80x83                        ⁢            lg            ⁢                                          1                                  1                  -                                                            "LeftBracketingBar"                                              S                        11                                            "RightBracketingBar"                                        2                                                              ⁡                              [                dB                ]                                                    ,                            (        2        )            
where
xcex93is the reflection attenuation in decibels.
In practice, the magnitude of the return attenuation is strongly dependent of the frequency used and, thereby, its degradation limits the frequency range desired by the user.
Another problem caused by an interface is the insertion loss occurring at the interface. In RF technology, it is often referred to by parameter S21. Its magnitude depends on the radiation losses at the interface, reflection attenuation and the different relative permittivities (xcex5r) of the materials on the different sides of the interface. Insertion loss also depends strongly on the frequency used since the permittivities (xcex5r) of materials change as the frequency becomes higher. Minimization of insertion losses is just as important as the minimization of the return attenuation in the desired frequency band if one wants to achieve good and low-loss transmission path matching at the interface.
Signal transmission paths in RF applications generally consist of coaxial conductors, striplines, microstrip conductors or coplanar conductors in various combinations. When looking for conductors that do not require much space or that can be planted on a substrate, one chooses either microstrip or coplanar conductors. The advantage of these conductors compared e.g. to coaxial cable is that they can be realized planar as far as the signal conductors are concerned. In the coplanar conductor structure, also the so-called ground conductor may be realized in the same plane with the signal conductor proper.
One way of matching the transmission line at the interface is to use a quarter-wave transformer shown in FIG. 1a, based on changing the width of the conductor in steps of xcex/4. A conductor 101 is placed on a suitable substrate 102. The width of the conductor is changed in four steps 103. However, the matching achieved in this way works only for a relatively narrow frequency band. The cause of this is the discontinuity that occurs at the steps 103, causing unwanted reactive fields or radiation into space at said steps 103.
Another widely used matching technique is so-called tapering. It means that the geometry of a conductor is changed by tapering it continuously for xc2xd to 1 xcex from original dimensions to desired dimensions, as shown in FIG. 1b. A conductor 104 is placed on a substrate 102. Tapering 105 of the conductor is realized without steps, i.e. continuously. Characteristic impedance matching realized by means of tapering is more controlled than impedance matching based on a quarter-wave transformer. Thus the unwanted phenomena occurring at the interface are smaller and the various losses will not increase together with the frequency as strongly as with a quarter-wave transformer.
In the publication xe2x80x9cIEEE Transactions on Components, Packaging and Manufacturing Technologyxe2x80x94Part B, vol 20, No. 1 February 1997, Decker and al, Multichip MMIC Package for X and Ka Bandxe2x80x9d there is presented a solution for realizing a more wideband matching for a feedthrough in a MMIC package. In that solution the transmission line matching is realized by tapering the conductor before taking it inside the MMIC package. The material of the wall of the MMIC package is an insulator the relative permittivity (xcex5r) of which is greater than the relative permittivity (xcex5r) of air. FIG. 2 illustrates the principle of the coupler arrangement thus realized. On top of the base structure 203 of the package there is a continuous ground plane 202 made of conductive material. On top of the ground plane there is a substrate 201 made of insulating material, and on top of the substrate there is a coplanar conductor structure, a signal conductor 204 and ground conductors 205. Near the conductors in the feedthrough there are also ground planes 206 which are connected through vias 209 to the ground plane under the substrate. The wall 208 of the package is made of insulating material as well. The characteristic impedance of the coplanar conductor changes as the conductors are taken into the wall of the package. Matching for the impedance change is realized by tapering 207. As seen from FIG. 2, tapering of the conductor is realized before the conductor is taken into the insulating material that the package walls consist of. Likewise, when the conductors come out of the wall material, another tapering 210 is realized which, too, is realized in free space. The feedthrough in the wall of a MMIC package according to this solution is applicable at up to 26 GHz, but not in the Ka band.
The return attenuation of the MMIC package feedthrough solution presented in the referenced document stays below xe2x88x9215 dB at up to 27.5 GHz. The insertion attenuation is of the order of 1 dB at up to 30 GHz, whereafter it grows rapidly.
In the publication Ishitsuka, T and Sato, N, Low Cost High-Performance Package for a Multi-Chip MMIC Modules, GaAs Symp. Dig. November 1988, pp. 221-224, there is presented another solution for a signal conductor feedthrough in a MMIC package. In that solution, the walls 208 of the MMIC package are comprised of multilayer ceramic sheets metallized on both sides. The ground potential planes resulting in the different layers are interconnected through several vias 209. The structure of the feedthrough of the signal conductor proper is otherwise like that described in the previously referenced document. This structure stretches the useable frequency band up to the 30 GHz limit. Disadvantages include the complexity of the wall structure and the resulting expensiveness of the structure.
The structures described in the publications mentioned above often employ GaAs-based chips. In GaAs ICs the coupling points of the signal conductors are located on the upper surface of the microchip, and the lower surface is covered by a continuous ground plane. When conductors according to the coplanar structure according to the above-referenced documents are connected to a GaAs circuit, the signal ground conductors must be taken from the upper surface of the GaAs circuit to the lower surface of the circuit. This is accomplished by making metallized vias on the GaAs chip. This complicates the structure of the IC and causes faulty connections as well as damaged chips in the manufacturing process.
An object of the invention is to reduce the above-mentioned disadvantages associated with the prior art. The matching method according to the invention for characteristic impedances is characterized in that the matching of a characteristic impedance is realized by tapering the conductor inside a wall made of dielectric material.
The matching method according to the invention for a characteristic impedance is characterized in that the matching of the characteristic impedance is realized by tapering the conductor inside a wall made of dielectric material.
The characteristic impedance coupler according to the invention is characterized in that the coupler comprises a wall made of dielectric material and therewithin a tapering with a first end and a second end, whereby a first signal line is coupled to the first end of said tapering and a second signal line is coupled to the second end of said tapering; and a first ground plane which is substantially parallel with the second signal line and at a first distance from the second signal line and which at least partly overlaps the second signal line as viewed from a direction perpendicular to the plane of the second signal line; and a second ground plane which is substantially parallel with the second signal line and at a second distance from the second signal line and which at least partly overlaps the second signal line as viewed from a direction perpendicular to the plane defined by the second signal line, whereby the second signal line lies between said first ground plane and second ground plane; and that said first distance and said second distance are substantially unequal.
An integrated circuit package according to the invention comprises a microcircuit that includes at least one coupling point and at least one grounding point. It is characterized in that the package comprises
a wall made of dielectric material,
a signal line a first end of which is located outside the package and a second end of which is located inside the package and the second end of which is coupled to a coupling point on the microcircuit through a coupling means, and
a ground plane coupled to said grounding point of the microcircuit; wherein characteristic impedance matching of the signal line is realized by tapering the signal line inside the wall made of dielectric material.
The basic idea of the invention is as follows: the matching of the conductor, either a microstrip or a coplanar conductor, coming to the MMIC package is realized inside the wall of the MMIC package. In the matching, the conductor is tapered and it is advantageously made an asymmetric strip conductor or coplanar conductor in conjunction with the tapering. Due to the asymmetricity of the conductor the electromagnetic field is concentrated in the lower part of the matching structure and the interface will not much change the shape of the propagating electromagnetic field.
An advantage of the invention is that the shape of the electromagnetic field changes only a little upon the transition from free space into the dielectric wall. As a result, the return attenuation of a matching structure according to an advantageous embodiment of the invention has in some simulations been below xe2x88x9210 dB at up to 40 GHz.
Another advantage of the invention is that the structure can be easily applied to taking signal conductors through MMIC package walls. Moreover, it is possible to reduce the number of feedthroughs realized in GaAs chips mounted in MMIC packages, because the lower ground plane in the structure according to the invention makes it possible to directly take the ground conductors onto the lower surface of the GaAs chip.
A further advantage of the invention is that the conductor matching structure is easy to realize using normal multilayer ceramic technology without having to resort to special techniques.